This application claims priority of Germany application 10134586.0 filed on Jul. 17, 2001.
The present invention relates to a sensor device for registering strain of a component.
There are numerous known methods for registering strain of a component, as induced by tensile, compressive, flexural and/or torsional forces. For example, for measuring the stress and strain on components subjected in particular to axial stress, such as pipes and bars, the strain is measured on the outer wall by means of strain gages. To achieve adequate signal strength and resolution, the cross section of the wall of the component is usually reduced in the measuring region in such a way that an elongation of 0.05 to 0.1% is achieved. Known examples are metal-film strain gages on a film of plastic, thin-film strain gages on steel, thick-film strain gages on steel or ceramic and magnetoresistive sensors. Even though all of these methods have their advantages, they also suffer from one or more disadvantages: disadvantages of metal-film strain gages on a film of plastic are creep of the adhesive, low endurance, high current consumption and difficult application. Thin-film strain gages on steel have the disadvantage of high production costs and complex contacting. Thick-film strain gages on steel or ceramic have only a low measuring accuracy and a short service life and, moreover, are suitable only for few grades of steel. In the case of magnetostrictive sensor devices, the measuring accuracy and service life are impaired by thermal influences and changes in the magnetic properties of the base body.
In general, it can be stated that the conventional sensor devices for registering strains do not satisfy the requirements in terms of measuring accuracy, service life, temperature independence and production and assembly costs that are appropriate for many intended applications.
DE 198 33 712 A1 discloses a pressure sensor device with a sensor chip which rests on a monolithically integrated structure. The sensor chip contains a plurality of sensor elements connected together in a measuring bridge and an analog or digital circuit required for the signal evaluation. The component to be measured is formed as a diaphragm body with a metal diaphragm, which is exposed on one side to the pressure medium and on the other side of which the sensor chip is attached by a glass of a low melting point, adhesive or the like. The strain of the metal diaphragm caused by the pressure of the pressure medium is registered by the sensor chip, to generate from this an electrical signal for the pressure to be measured. It is disadvantageous that the production of the metal diaphragm on the measuring body requires corresponding effort. In addition, the attachment of the silicon sensor chip on the metal diaphragm involves considerable difficulties. With adhesives in particular, it is not possible for the sensor chip to be mounted on the metal diaphragm without any creep and with long-term stability. In addition, the calibration of the sensor chip attached to the metal diaphragm requires considerable effort.
In one embodiment of the present invention, a sensor device is provided for registering strain of a component which, in spite of lowest possible production and assembly costs, has a high measuring accuracy over a relatively large temperature range and a long service life. In particular, it is intended to be suitable for use in the motor vehicle sector.
In an embodiment of the present invention, a sensor device comprises a sensor chip and a supporting substrate, which are connected to each other durably and without creep such that the sensor chip, the supporting substrate and the connecting layer form a prefabricated sensor module. The sensor module can be attached to the component to be measured by a durable and creep-resistant connection such that the strain of the component is transferred from the supporting substrate to the sensor chip, which then generates a corresponding electrical output signal.
In a preferred embodiment of the present invention, the sensor chip has a small thickness of about, for example, 20 to 100 xcexcm. It is applied by a connecting process, such as for example glass or AuSn soldering, to the supporting substrate, which includes for example of steel or an NiFeCo alloy. This produces a sensor module in the form of a very stable layered structure which forms a functional unit with long-term stability, no creep and little hysteresis.
In one aspect of the invention, this functional unit permits a high measuring accuracy (of for example 0.25%) over a relatively great temperature range (of for example xe2x88x9240 to +140xc2x0 C.), has a long service life, is inexpensive to produce and can be mounted in a simple way on the component to be measured, and can be calibrated already before it is attached to the component to be measured.